Field of the Invention
The present invention mainly relates to a radar device with which radar images of a plurality of display ranges can be displayed either simultaneously or one by one.
Background Information
Radar images of different display ranges are used according to the objective at hand in marine radar devices and the like. For example, if the user wants to check what the exact situation is around his vessel, he uses a radar image with a narrow display range. If the user wants to check on the situation farther away, he uses a radar image with a wide display range.
With a radar device of this type, the pulse width of the pulse signals that will be transmitted is determined ahead of time according to the display range of the radar images. For instance, if a radar image has a narrow display range, a pulse signal is transmitted at a relatively narrow pulse width. This allows the distance resolution to be increased, and therefore is useful when the user wants to check on the exact situation near his vessel.
On the other hand, if the display range is wide, the signal-to-noise power ratio of the echo signal from a target may decrease, which would lower the visibility of the echo image on the radar image. In view of this, when the display range of a radar image is wide, a pulse signal is transmitted at a relatively wide pulse width. Consequently, the pulse width of the echo from the target will also be wider, so the signal-to-noise power ratio of the echo signal from the target will increase, and the visibility of this echo image will be better.
As discussed above, with a conventional radar device, the pulse width of a transmitted pulse signal varies with the display range.
Japanese Laid-Open Patent Application Publication No. 2007-298417 (Patent Literature 1) discloses a radar device featuring a “dual-screen simultaneous display mode” in which two radar images of different display ranges are produced and simultaneously displayed.
As shown in FIG. 3(a), for example, the configuration of Patent Literature 1 is such that a pulse signal for use with a display range R1 and a pulse signal for use with a display range R2 are transmitted alternately. A radar image for the display range R1 is produced on the basis of the echo obtained during the transmission and reception period of the R1 pulse signal, and a radar image for the display range R2 is produced on the basis of the echo obtained during the transmission and reception period of the R2 pulse signal. The proper radar image for each display range can be produced by properly setting the pulse width for the R1 and R2 pulse signals according to the respective display ranges.
Apart from this, there is a known pulse compression radar device. With pulse compression radar, a pulse signal that has undergone frequency modulation (modulated pulse) is transmitted, and the echo received from the surroundings is subjected to pulse compression processing, which enhances the S/N ratio.
From the standpoint of improving the S/N ratio after pulse compression, it preferable for the transmitted modulated pulse to have a wider pulse width. If the pulse width of the modulated pulse is increased, however, a problem is that there is a larger blind zone (in which echoes cannot be received from short distances) during the transmission and reception period of this modulated pulse.
In view of this, an unmodulated pulse (a pulse signal that has not undergone frequency modulation) with a narrow pulse width is sometimes used in order to compensate for the blind zone of the modulated pulse in a pulse compression radar device. For example, as shown in FIG. 3(b), an unmodulated pulse with a narrow pulse width and a modulated pulse with a wide pulse width are transmitted alternately, and a composite echo signal is produced by synthesizing an echo signal from a short distance obtained during the transmission and reception period of the unmodulated pulse and an echo signal from farther away obtained during the transmission and reception period of the modulated pulse. A radar image that is a combination of a short-distance radar image based on an unmodulated pulse and a far radar image based on a modulated pulse can be obtained, as shown in FIG. 4, by producing radar images on the basis of this composite echo signal. This allows targets to be detected at short to long distances.
With a pulse compression radar configured as above, since an echo from a short distance is an unmodulated echo, pulse compression offers no increase in S/N ratio. Therefore, echoes from a short distance are relatively susceptible to the effect of noise, but they also have a high signal level to start with, and tend not to be buried in noise, so there is no practical problem.